The spread spectrum technique is based on the interchangeability of signal/noise ratio and bandwidth. Spread spectrum systems (SSS) employ an auxiliary function for spectrum spreading. The waveform of this function is known to the receiver. This results in the highly advantageous property that these systems can operate even under very difficult signal/noise ratios. The auxiliary functions employed may be signals which themselves have a large bandwidth and transmit this to the transmission signal when linked with the information signal. Due to the large product of time x bandwidth an advantageous autocorrelation function (AKF=) may be produced, with a sharp peak at the origin and low side lobe values, and the necessary synchronization of the code sequences forming the auxiliary functions may be produced in the receiver by means of this property.
Another property of SSS is that several spread spectrum signals can be transmitted simultaneously in one and the same channel of a given bandwidth under the condition that the auxiliary functions of different users differ distinctly in their cross-correlation properties. It is thereby possible to realise networks with multiple access (Code
Division Multiple Access=CDMA). The nature of the spectrum of SSS to a certain extent allows operation on frequency bands which are already in use by narrow band services without significant interference between these two systems. Moreover, by using pseudorandom code sequences with short so called chip duration Tc (=smallest rectangular impulse duration of the auxiliary function), it is possible to resolve individual radio signals that are propagated over several natural paths (so called multipath connections), and utilize them effectively as diversity components.
This invention relates to a digital radio transmission system for a network built up of cells, using the spread spectrum technique, in which spreading of the spectrum is achieved at the transmission end by multiplication of the information carrying signal with an auxiliary function while despreading is brought about at the receiving end by utilizing the same auxiliary function, the system comprising several user stations within a cell, each station equipped with a transmitter, a receiver for multipath reception and a control unit, and further comprising a base station with a plurality of transmitter-receivers and a base control unit.
Systems of this type, in which spectrum spreading is brought about by multiplication of the information carrying signal with the auxiliary function, are known as Direct Sequence-Spread-Spectrum Systems (DS-SSS) (see, for example, R. C. Dixon: "Spread Spectrum Systems", John Wiley Interscience, 1984). The methods known in the literature for the realization of DS-SSS receivers (A. Baier: "A Low-Cost Digital Matched Filter for Arbitrary Constant-Envelope Spread-Spectrum Waveforms" IEEE Trans. Comm., Vol. COM-32, April 1984, page 354; M. Kowatsch: "Synchronisation in a Spread-Spectrum Communication Modem Based on SAW Convolvers", IEEE Milcom '84, Los Angeles, October 1984, page 9.5. 1; U.S. Pat. No. 4 672 658) may be roughly divided into two categories, the matched filter type and the correlation type receiver. Both processes aim at highly accurate synchronization auxiliary sequence in the receiver, with the transmission auxiliary sequence. The main parameter for the first type is the time x bandwidth product of the matched filter Nowadays, sufficiently high values can only be achieved with CCD (Charged Coupled Devices) and SAW (Surface Acoustic Wave) technologies. The advantage of the matched filter receiver is the rapid synchronization but the great disadvantage is the limitation of process gain of this technology, i.e. the limited correlation time and hence the period duration of the auxiliary function. The SAW technology has the disadvantage of a small dynamic range due to its high intrinsic losses while CCD are limited in the clock frequency and digital matched filters are not optimal due to the amplitude quantization and the chip surface required.
The correlation type receiver, on the other hand, has the disadvantage of a longer synchronization time, although this is not found to be a disadvantage in most applications. During the process of synchronization, the receiver code is shifted continuously or stepwise according to the time delay to the transmitter and correlated with the received signal until the maximum of the correlation function has been found, i.e. the residual shift is smaller than the chip duration. This search process is dependent upon the length M of the auxiliary function but there are no restrictions to the period duration of the auxiliary function.
A rough principle of operation is already known from the above literature by R.C. Dixon. It is also known that the receiver structure can be extended so that several correlations with displaced auxiliary functions can take place simultaneously. This leads to somewhat more rapid acquisition (finding of the synchronous moment in time) and when there are several propagation paths, these may each be correlated synchronously independently of one another so that the natural diversity of the different paths can be utilized. H. Ochsner describes such an architecture in "An Antimultipath Spread-Spectrum Receiver and its Application to Portable Radio Telephone", IEEE Globecom '86, Houston, December 1986, page 31.7.1, but without giving any practical details for realizing such a system. Ochsner also fails to disclose any strategy for initial synchronization and its maintenance.